Rotten Ghouls are ghouls that guard or were just raised together with Leprithus. They can be found in the human territory of Westfall.
They spawn several places;
Under a tree a little south west of the Jangolode Mine. The tree has a small square fence about it with some coffins within the fence line. Here they wont move from.
in the Dagger Hills, directly to the north and slightly to the east of the Deadmine's gates (in Stranglethorn Vale). There is another tree with coffins under it, here they will path back and forth from this location to the bridge to Duskwood, east of The Dead Acre.